Venice continues to sink slowly

Washington: Though previous studies had found that Venice has stabilized, new measurements indicate that the historic city continues to slowly sink, and even to tilt slightly to the east.Researchers revealed that the city is subsiding on average of 2 mm per year.

"Venice appears to be continuing to subside, at a rate of about 2 millimeters a year," said Yehuda Bock, a research geodesist with Scripps Institution of Oceanography at the University of California, San Diego, in La Jolla, Calif., and the lead author of the new article on the city's downward drift."It's a small effect, but it's important," he added.Given that sea level is rising in the Venetian lagoon, also at 2mm per year, the slight subsidence doubles the rate at which the heights of surrounding waters are increasing relative to the elevation of the city, he noted.In the next 20 years, if Venice and its immediate surroundings subsided steadily at the current rate, researchers would expect the land to sink up to 80 mm (3.2 inches) in that period of time, relative to the sea.Bock worked with colleagues from the University of Miami in Florida and Italy's Tele-Rilevamento Europa, a company that measures ground deformation, to analyze data collected by GPS and space-borne radar (InSAR) instruments regarding Venice and its lagoon.The GPS measurements provide absolute elevations, while the InSAR data are used to calculate elevations relative to other points. By combining the two datasets from the decade between 2000 and 2010, Bock and his colleagues found that the city of Venice was subsiding on average of 1 to 2 mm a year (0.04 to 0.08 inches per year).The patches of land in Venice’s lagoon (117 islands in all) are also sinking, they found, with northern sections of the lagoon dropping at a rate of 2 to 3 mm (0.08 to 0.12 inches) per year, and the southern lagoon subsiding at 3 to 4 mm (0.12 to 0.16 inches) per year."Our combined GPS and InSAR analysis clearly captured the movements over the last decade that neither GPS nor InSAR could sense alone" said Shimon Wdowinski, associate research professor of Marine Geology and Geophysics at the University of Miami.